feat: upload job summary when supported (#917)

- Add GitHub-style Actions **job summaries** support (writes to `GITHUB_STEP_SUMMARY` / `workflow/SUMMARY.md`) and render them in the run UI.
- Gitea stores summaries internally (DB) and serves them in the run view payload.
- `act_runner` uploads the summary **only when Gitea advertises support** (`X-Gitea-Actions-Capabilities: job-summary`), and warns on upload failures without failing the job.

## Compatibility
- New Gitea + old runner: no upload → no summary shown (no behavior change)
- New runner + old Gitea: capability not advertised → runner skips upload (no behavior change)

## Issue
- Fixes go-gitea/gitea#23721

Reviewed-on: https://gitea.com/gitea/runner/pulls/917
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

internal/app/cmd/daemon.go

@@ -148,6 +148,7 @@ func runDaemon(ctx context.Context, daemArgs *daemonArgs, configFile *string) fu
 			log.Infof("runner: %s, with version: %s, with labels: %v, declare successfully",
 				resp.Msg.Runner.Name, resp.Msg.Runner.Version, resp.Msg.Runner.Labels)
 		}
+		runner.SetCapabilitiesFromDeclare(resp)
 
 		if cfg.Metrics.Enabled {
 			metrics.Init()